Yesterday we noticed this gall on our largest protected California Black Walnut Tree, and did some internet research. We can’t locate a convincing photo, but believe it might be the result of the Velvet Gall Mite, Eriophyes caulis. We will check with local California Black Walnut experts Clare Marter-Kenyon and Julian Donahue to see if they know of this mite on Los Angeles trees. According to the information we can locate: “Little is known about the mites that occur on black walnut, but the velvet gall mite is common in some areas. The mite itself is so small that it cannot be seen with the unaided eye. Injury The velvet gall mite causes a conspicuous velvety red growth up to an inch long on the leaf stem, often causing the leaf to curl or twist over on itself. Galls may be numerous on individual trees but they are considered to be harmless to the tree. Control No control is recommended. ”

Location: Mt Washington

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